1. Field of the Invention
This invention relates to a process for producing a denitration catalyst. More particularly it relates to a process for producing a denitration catalyst suitable for catalytically reducing nitrogen oxides contained in exhaust gases of a low dust content with ammonia. The exhaust gases of a low dust content refers to those discharged from combustion of gas or liquid fuels such as LPG, naphtha, light oil and the like petrorium fractions.
2. Description of the Related Art
Nitrogen oxides (NOx) in exhaust gases discharged from various boilers and the like are atmosphere-pollutive substances together with sulfur oxides (SOx).
As processes for removing the NOx, various ones have been proposed and among these, a process of selectively reducing NOx with ammonia added to exhaust gases in the presence of a catalyst has been mainly utilized. As such catalyst, various catalysts using titanium oxide as a base compound have been proposed (Japanese patent application laid-open No. Sho 50-128681/1975 and No. Sho 53-28148/1978) and have currently been practically used.
In the case where exhaust gases containing no dust such as LPG combustion gases are treated, a highly active and economical catalyst is required; thus besides those obtained by molding a catalyst itself into a honeycomb form or a spherical form, those obtained by coating catalyst compounds onto a ceramics honeycomb-form substrate or a metal substrate have been proposed to reduce the quantity of the catalyst compounds used. However, a catalyst obtained by merely coating catalyst compounds onto a metal substrate has a drawback that it has a low catalyst strength and a small quantity of the catalyst compounds supported there on so that it is difficult to retain its high activity.
In order to overcome the drawback, various processes have been proposed such as a process of improving the catalyst strength by adding various binders to catalyst compounds (Japanese patent application laid-open No. Sho 49-58093/1974), a process of roughening the metal surface, thereby increasing the quantity of the catalyst compounds supported thereon to obtain a high activity of the catalyst (Japanese patent application laid-open No. Sho 51-116168/1976), etc. However, such problems have been raised that when binders are added to improve the catalyst strength, the catalyst activity lowers, while when the metal surface is roughened and increased with the catalyst compounds supported thereon, the catalyst compounds are liable to peel off from the substrate.
Further, in the case where a sheet-form catalyst obtained by having catalyst compounds supported on a metal plate substrate is used, it has been required to make the plate catalyst thinner than conventional catalysts so that the distance (clearance) between the plate catalysts provided inside the flow path of exhaust gases can be narrowed to make the catalyst apparatus compact and reduce the quantity of the catalyst compounds used, thereby saving the cost. Further, it is desired that the catalyst performance should not be lowered than that of a conventional catalyst.